Water is the most important constituent of plants and is essential for the maintenance of life, growth and development.

Plants lose huge amount of water through transpiration.

They have to replenish this lost water to prevent wilting.

Water is mainly absorbed by the roots of the plants from the soil, than it moves upward to different parts and is lost from the aerial parts, especially through the leaves.

Before taking up the absorption and movement of water in plants, it is worthwhile to understand the phenomenon of imbibition, diffusion and osmosis involved in the water uptake and its movement in the plants.

Imbibition

The process of adsorption of water by solid particles of a substance without forming a solution is called 'imbibition'.

It is a type of diffusion by which movement of water take place along a diffusion gradient.

The solid particles which adsorb water or any other liquid are called imbibants. The liquid which is imbibed is known as imbibate.

Cellulose, pectic substances, protoplasmic protein and other organic compound in plant cells show great power of imbibition.

(i) Characteristics of Imbibition

The phenomenon of imbibition has three important characteristics :

(a) Volume change

During the process of imbibition, imbibants increase in volume.

It has been observed that there is an actual compression of water. This is due to arrangement of water molecules on surface of imbibant and occupy less volume than the same molecules do when are in free stage in the normal liquid.

During the process of imbibition affinity develops between the adsorbant and liquid imbibed.

A sort of water potential gradient is established between the surface of adsorbant and the liquid imbibed.

For Example: If a dry piece of wood is placed in water, it swells and increases in its volume. Similarly, if dry gum or pieces of agar-agar are placed in water, they swell and their volume increases.

Wooden doors and windows adsorb water in humid rainy season and increase in their volume so that they are hard to open or close, in gram and wheat the volume increase by adsorption of water, in plant systems are adsorption of water by cell wall.

(b) Production of heat

As the water molecules are adsorbed on the surface of the imbibant, their kinetic energy is released in the form of heat which increase the temperature of the medium. It is called heat of wetting (or heat of hydration). e. g., during kneading, the flour of wheat gives a warm feeling due to imbibition of water and consequent release of heat.

(c) Development of imbibition pressure

If the imbibing substance (the imbibant) is confined in a limited space, during imbibition it exerts considerable pressure.

The bursting of seed coats of germinating seeds is the result of imbibition pressure developed within the seeds as they soak the water.

Imbibition pressure can be defined as the maximum pressure that an imbibant will develop when it is completely soaked in pure water. Imbibition pressure is also called as the matrix potential because it exists due to the presence of hydrophilic substances in the cell which include organic colloids and cell wall.

Resurrection plants of Selaginella, lichens, velamen roots and dry seeds remain air dry for considerable periods because they can absorb water from the slight downpour by the process of imbibition.

(ii) Factors Influencing the Rate of Imbibition

(a) Nature of imbibant : Proteins are the strongest imbibants of water, starch less strong, cellulose being the weakest. That is why proteinaceous pea seeds swell more than the starchy wheat seeds. During seed germination seed coat rupture first because it is made up of cellulose (weak imbibant) and kernel is made up of protein, fat and starch (strong imbibant).

(b) Surface area of imbibant : If more surface area of the imbibant is exposed and is in contact with liquid, the imbibition will be more.

(c) Temperature : Increase in temperature causes an increase in the rate of imbibition.

(d) Degree of dryness of imbibant : If the imbibant is dry it will imbibe more water than a relatively wet imbibant.

(e) Concentration of solutes : Increase in the concentration of solutes in the medium decreases imbibition due to a decrease in the diffusion pressure gradient between the imbibant and the liquid being imbibed. It is due to the fact that imbibition is only a special type of diffusion accompanied by capillary action. If some solute is added into the liquid which is being imbibed, its diffusion pressure decreases and the process of imbibition slows down.

(f) pH of imbibant : Proteins, being amphoteric in nature, imbibe least in neutral medium. Towards highly acidic or highly alkaline pH, the imbibition increases till a maximum is reached, there after it starts slowing down.

(iii) Significance of imbibition

(a) The water is first imbibed by walls of root hairs and then absorbed and helps in rupturing of seed coat (made up of cellulose).

(b) Water is absorbed by germinating seeds through the process of imbibition.

(c) Germinating seeds can break the concrete pavements and roads etc.

(d) The water moves into ovules which are ripening into seeds by the process of imbibition.

(e) It is very significant property of hydrophilic surfaces.

Diffusion

The movement of the molecules of gases, liquids and solutes from the region of higher concentration to the region of lower concentration is known as diffusion.

Or Diffusion is the net movement of molecules or ions of a given substance from a region of higher concentration to lower one by virtue of their kinetic energy.

Or It is the movement of molecules from high diffusion pressure to low diffusion pressure.

Phenomenon of diffusion can be observed every day.

It may occur between gas and gas (e.g., diffusion of ammonia into air), liquid and liquid (e.g., diffusion of alcohol into water), or solid and liquid (e.g., diffusion of sugar into water).

The diffusion of one matter is dependent on other. That is why many gases and solutes diffuse simultaneously and independently at different rates in different direction at the same place and time, without interfering each other.

From soil, water and ions of simple inorganic salts pass into plants through the root cells by a process which is basically diffusion, though greatly modified by other factors.

The water and solutes pass through the dead and living vessels and also from cell to cell by diffusion.

When a crystal of copper sulphate is placed in a beaker containing water, a dense blue colour is seen around the crystal.

Diffusion pressure

It is a hypothetical term coined by Meyer (1938) to denote the potential ability of the molecules or ions of any substance to diffuse from an area of their higher concentration to that of their lower concentration. Alternatively, it may also be defined as the force with which the diffusing molecules move along the concentration gradient.

Diffusion Pressure Deficit (DPD) or Suction Pressure (SP)

The term diffusion pressure (DP) and diffusion pressure deficit (DPD) were putforth by B.S. Meyer in 1938. Originally, the DPD was described by the term suction force (Saugkraft) or suction pressure (SP) by Renner (1915). Now a days, the term water potential (y) is used which is equal to DPD, but negative in value.

Each liquid has a specific diffusion pressure. Pure water or a pure solvent has the maximum diffusion pressure.

If some solute dissolved in it, the water or solvent in the resulting solution comes to attain less diffusion pressure than that of the pure water or pure solvent. In other words, diffusion pressure of a solvent, in a solution is always lower than that in the pure solvent.

'The amount by which the diffusion pressure of water or solvent in a solution is lower than that of pure water or solvent is known as diffusion pressure deficit (DPD)'.

Because of the presence of diffusion pressure deficit, a solution will always tend to make up the deficit by absorbing water. Hence, diffusion pressure deficit is the water absorbing capacity of a solution. Therefore, DPD can also be called suction pressure (SP).

(iii) Factors Influencing Rate of Diffusion

(a) Temperature : Increase in temperature leads to increase in the rate of diffusion.

(b) Pressure : The rate of diffusion of gases is directly proportional to the pressure. So the rate of diffusion increases with increase of pressure. Rate of diffusions µ pressure.

(c) Size and mass of diffusing substance : Diffusion of solid is inversely proportional to the size and mass of molecules and ions.

Rate of diffusion µ 1/Size X mass of particles

(d) Density of diffusing substance : The rate of diffusion is inversely proportional to the square root of density of the diffusion substance. Larger the molecules, slower will be the rate of diffusion. This is also called Graham's law of diffusion.

D µ 1/√d (D = Diffusion and d = Density of diffusing substance).

According to the density the diffusion of substances takes place in following manner–

Gas > Liquid > Solid

The vapours of volatile liquids (sent or petrol) and solids (camphor) also diffuse like gases.

(e) Density of the medium : The rate of diffusion is slower, if the medium is concentrated. Thus, a gas would diffuse more rapidly in vacuum than in air. Substances in solution also diffuse but at a much slower rate than gases. Substances in solution diffuse more rapidly from regions in which their concentration is higher into regions of low concentration. If two solutions of sugar (or of any other substance) of different concentrations are in contact, sugar molecules diffuse from the higher to the lower concentrations of sugar and water molecules diffuse from the higher to the lower concentrations of water, until equilibrium is attained when the two solutions become of equal concentration.

(f) Diffusion pressure gradient (DPG) : The rate of diffusion is directly proportional to the difference of diffusion pressure at the two ends of a system and inversely proportional to the distance between the two.

(iv)Significance of Diffusion

Gaseous exchange during the processes of photosynthesis and respiration takes place with the help of diffusion.

The process of diffusion is involved in the transpiration of water vapours.

Aroma of flowers is due to diffusion of volatile aromatic compounds to attract pollinating animals.

During passive salt uptake, the ions are absorbed by process of diffusion.

Diffusion helps in translocation of food materials.

Gaseous exchange in submerged hydrophytes is takes place by general surface of the cells through diffusion.

Permeability

Permeability is the degree of diffusion of gases, liquids and dissolved substances through a membrane. Different types of membranes may be differentially permeable to different substances. Normally, permeability of a given membrane remains unchanged, but it may change with alteration in the environmental conditions of the cell.

(i) Types of Membranes on the Basis of Permeability

(a) Permeable membrane : These membranes allow free passage of solvent (water) and most of the dissolved substances. e.g., cell wall in plant cells. Filter paper is made up of pure cellulose it also functions as permeable membrane.

(b) Impermeable membrane : This type of membranes with deposite of waxy substances like cutin and suberin, do not allow the entry of water, dissolved substances and gases. e.g., suberized walls of cork cells, cuticle layer of leaf.

(d) Selectively or Differentially permeable membrane :This type of membranes allow selective passage of solutes along with solvent, through them. e.g., Osmotic diffusion of water through selectively permeable membrane start from higher water potential to lower water potential. Many biological membranes such as cell membrane (plasmalemma), tonoplast (vacuolar membrane) and the membranes surrounding the sub-cellular organelles are selectively permeable. A non-living selectively permeable membranes is cellophane.

Osmosis

Osmosis (Gr. Osmos = a pushing or impulse) was discovered by Abbe Nollet in 1748 and also coined the term 'osmosis'. First of all Traube (1867) use copper ferrocyanide and develop semipermeable membrane to show its utility in the osmosis of plant physiology. First time Pfeffer in (1887) develop osmoscope by using semipermeable membrane.

Osmosis is special type of diffusion of a liquid, when solvent moves through a semipermeable membrane from a place of higher diffusion pressure to a place of lower diffusion pressure

It is the migration of solvent from a hypotonic solution (of lower concentration) to hypertonic solution (of higher concentration) through a semi-permeable membrane to keep the concentration equal.

In formalin preserved Spirogyra filament, selective permeability of plasmamembrane is lost and hence no effect on placing in hypertonic solution.

If salt presents in higher concentration in a cell than outer side, water will enter in the cell by osmosis.

In osmosis, the water (or solvent) molecules moves as follows :

From the region of

To the region of

Pure solvent (water)

Solution

Dilute solution

Concentrated solution

High free energy of water molecules

Low free energy of water molecules

Higher chemical potential (or water potential)

Lower chemical potential (or water potential)

Higher diffusion pressure of water

Lower diffusion pressure of water

Differences between diffusion and osmosis

S.No.

Diffusion

Osmosis

(1)

It is the movement of particles, molecules or ions from the region of their higher free energy to the region of their lower free energy.

It is the movement of solvent of water from the area of its higher free energy or chemical potential to the area of its lower free energy or chemical potential through a semi-permeable membrane.

(2)

It can occur in any type of medium.

It occurs only in liquid medium.

(3)

The diffusing molecules may be solids, liquids or gases.

It involves the movement of solvent molecule only.

(4)

It does not require a semi-permeable membrane.

A semi-permeable membrane is required for the operation of osmosis.

(5)

It is purely dependent upon the free energy of the diffusing substance only.

It depends upon the free energy chemical potential of the solvent present on the two sides of the semi-permeable membrane.

(6)

An equilibrium in the free energy of diffusing molecules is achieved in the end.

An equilibrium in the free energy of solvent molecule is never achieved.

Osmotic pressure (OP)

Pfeffer coined the term osmotic pressure.

Osmotic pressure of a solution is the pressure which must be applied to it in order to prevent the passage of solvent due to osmosis.

Osmotic pressure is that equivalent of maximum hydrostatic pressure which is produced in the solution, when this solution is separated from its pure solvent by a semipermeable membrane.

It can also be defined as "the excessive hydrostatic pressure which must be applied to it in order to make its water potential equal to that of pure water". Osmotic pressure is equal to the pressure which is needed to prevent the passage of pure water into an aqueous solution through a semi-permeable membrane. In other words, it is that pressure which is needed to check the process of osmosis.

Significance of osmosis in plants

(a) The phenomenon of osmosis is important in the absorption of water by plants.

(b) Cell to cell movement of water occurs throughout the plant body due to osmosis.

(c) The rigidity of plant organs (i.e., shape and form of organism) is maintained through osmosis.

(d) Leaves become turgid and expand due to their OP.

(e) Growing points of root remain turgid because of osmosis and are thus, able to penetrate the soil particles.

(f) The resistance of plants to drought and frost is brought about by osmotic pressure of their cells.

(g) Movement of plants and plant parts, for example, movement of leaflets of Indian telegraph plant, bursting of many fruits and sporangia, etc. occur due to osmosis.

(h) Opening and closing of stomata is affected by osmosis.

Water Potential (Slatyer and Taylor, 1960) (y)

The movement of water in plants cannot be accurately explained in terms of difference in concentration or in any other linear expression.

The best way to express spontaneous movement of water from one region to another is in terms of the difference of free energy of water between two regions.

Free energy is the thermodynamic parameter, that determine the direction in which physical and chemical changes must occur.

The potential energy of water is called water potential. e.g., water is stored behind a dam.

When the water runs downhill, its potential energy can be converted to electrical energy. This conversion of energy of water is due to gravity.

The other source that provides energy to water is pressure. The increasing pressure increases the free energy there by increasing water potential.

Osmotic Or Solute Potential (Ψs)

It is the decrease in the chemical potential of pure water due to the presence of solute particles in it. It is a colligative property of solute and is dependent upon the number of solute particles and not upon the nature of solute. Solute particles reduce the free energy of water by diluting it, increasing entropy, reducing vapour pressur, raising thr free energy of water by diluting it, increasing entropy, reducing vapour pressure, raising boiling point and lowering freezing point. Its value is calculated by the following formula

OP or SP = C×R×T

Pressure Potential (Ψp) or Hydrostatic Pressure

It is the pressure which develops in an osmotic system due to osmotic entry or exit of water from it.

A positive pressure develops in a plant cell or system due to entry of water into it. Positive hydrostatic pressure is also called turgor pressure.

Loss of water produces a negative hydrostatic pressure or tension.

It develops in xylem due to loss of water in transpiration. This is very important in transport of sap over long distances in plants.

Osmotic Relations of Plant Cells

A plant cell functions as an osmotic apparatus or osmotic system.

It is bathed in a water medium or lies in contact with other cells having water.

A typical plant cell has a permeable elastic wall, a semipermeable membrane (plasmalemma or plasma membrane alone or alongwith cytoplasm and tonoplast) and an osmotically active solution called cell sap (contained in the central vacuole).

It causes the osmotic entry of water which develops a turgor pressure (TP) or pressure potential (Ψp).

Plasmolysis

(Gr. Plasma = something formed; lysis = loosing)

If a living plant cell is placed in a highly concentrated solution (i.e. hypertonic solution), water comes out of the cell due to exosmosis, through the plasma membrane.

The loss of water from the cell sap causes shrinkage of the protoplast away from the cell wall in the form of a round mass in the centre.

"The shrinkage of the protoplast of a living cell from its cell wall due to exosmosis under the influence of a hypertonic solution is called plasmolysis".

The stage of plasmolysis, when the protoplast just begins to contract away from the cell wall is called incipient plasmolysis.

The stage when the cell wall has reached its limit of contraction and the protoplast has detached from cell wall attaining spherical shape is called evident plasmolysis.

In a plasmolysed cell, the space between the contracted protoplast and the cell wall remains filled with external solution.

If a cell with incipient plasmolysis is placed in a hypertonic solution it will show more plasmolysis.

If a plasmolysed cell is placed in pure water or hypotonic solution, endosmosis takes place.

The protoplast attains its original shape and the cell regains its original size.

"The swelling up of a plasmolysed protoplast due to endosmosis under the influence of a hypotonic solution or water is called deplasmolysis'.

Deplasmolysis is possible only immediately after plasmolysis otherwise the cell protoplast becomes permanently damaged. Leaf of Tradescantia is used for demonstration of plasmolysis in laboratory.

The value of TP becomes zero at the time of limiting plasmolysis and below zero during incipient and evident plasmolysis.

Significance of Plasmolysis

It proves the permeability of the cell wall and the semipermeable nature of the protoplasm.

The OP of a cell can be measured by plasmolysis. The OP of a cell is roughly equal to the OP of a solution that causes incipient plasmolysis in the cell.

Salting of pickles, meat, fishes etc. and addition of sugar to jams, jellies, cut fruits etc., prevent their decay by microbes, as the latter get killed due to plasmolysis or due to high concentration of salt or sugar.

By salting, the weeds can be killed from tennis courts and the growth of plants can be prevented in the cracks of walls.

Plasmolysis is helpful in determining whether a particlular cell is living or dead as plasmolysis does not occur in a dead or non-living cell.

Q.1 Movement of particles of matter in the cell due to its own kinetic energy is called